Carotid endarterectomy (CEA) is extremely efficacious in preventing stroke- and reducing death due to the low incidence of major surgical complications (2-5 percent incidence of stroke). However, at least 30 percent of endarterectomy patients experience cognitive deficits, revealed by a battery of neuropsychometric tests (NPMTs). Although most strokes after CEA are due to FOCAL injury from emboli coming from the surgical field, we hypothesize that some patients without EEG evidence of cerebral ischemia may still have GLOBAL ischemia as indicated by TCD that leads to cognitive deficits and decreases quality of life. We further hypothesize that the incidence and severity of cognitive deficits will be significantly reduced in these patients if CBF is augmented by placement of a shunt across the operative site. Specific Aims: In order to test thehypotheses we propose the following: 1. To demonstrate that a certain percentage of patients with normal EEGs develop significant cognitive deficits due to GLOBAL brain parenchymal damage after CEA by three independent measures of cerebralinjury ury:functional, structural and biochemical 2. To determine prospectively the importance of GLOBAL hypoperfusion by randomizing patients who have normal EEGs but significantly teduced CBF velocity after carotid artery clamping into two groups. One group will have a shunt inserted across the operative site and the other, not (the current standard). Each will be assessed for cognitive deficits, and structural and biochemical changes. Conclusion: We shall demonstrate that cognitive deficits arise from GLOBAL ischemia when the carotid artery is clamped, which can be assessed by TCD ultrasonography and prevented by increasing cerebral perfusion using a shunt across the surgical site. EEG is inadequate to determine this subtle degree of hypoperfusion.